1. Field of the Invention
The disclosures generally relate to an electrochromic display element, a display device and an information apparatus, and more specifically relate to a reflective electrochromic display element, a display device having such an reflective electrochromic display element and an information apparatus having such a display device.
2. Description of the Related Art
Electronic paper has been increasingly developed as information media replacing ordinary paper. Such electronic paper is generally formed of a display element. It is desirable that the display element utilized for the electronic paper be (1) a reflective display element, include (2) high white reflectance, (3) a high contrast ratio, (4) capability of exhibiting high-definition display, (5) capability of exhibiting memory effects and (6) capability of being driven by a low voltage, and be (7) light and thin, and (8) inexpensive. In particular, it is desirable that the display element utilized for the electronic paper include the white reflectance and the contrast ratio as high as those of the ordinary paper.
Various types of display elements have been proposed for use in the electronic paper. Examples of the proposed display elements include a reflective liquid crystal display element utilizing a reflective liquid crystal, an electrophoretic display element utilizing electrophoresis and a display element utilizing toner migration. However, the proposed display elements may not be able to exhibit a multicolor display while maintaining the white reflectance and the contrast ratio as high as those of the ordinary paper.
In general, display elements include different color filters for displaying multiple color. However, since the color filters themselves absorb light, the display elements provided with the color filters lower their reflectance. Further, since each pixel is divided into three color parts of Red (R), Green (G) and Blue (B) in the display element provided with the color filters, the reflectance of the display element may significantly be lowered.
If the white reflectance and the contrast ratio of the display element are significantly lowered, the visibility of the display element may be significantly lowed. Hence, the display element may not be suitable for use in the electronic paper.
As a potential reflective display element, a technology utilizing “electrochromism” has been put forward.
Electrochromism is a phenomenon displayed by specific materials of reversibly changing optical properties when an amount of voltage is applied to the materials. Most general examples of such specific materials exhibiting “electrochromism” include chemical compounds, which induce a reversible redox reaction to reversibly change their colors when a voltage is applied to the chemical compounds. The specific materials exhibiting “electrochromism” are hereinafter called “electrochromic compounds”.
Electrochromid display elements are defined as display elements utilizing color development/color dissipation of the electrochromic coumpounds.
The electrochromic display element is a reflective display element, has memory effects, and capable of being driven by a low voltage. Accordingly, the electrochromic display element has been extensively studied as one of the promising candidates for display devices utilized for the electronic paper, from material development to device design.
Further, since the electrochromic display elements are capable of developing various colors based on various structures of electrochromic compounds, the electrochromic display elements are expected to be utilized as multicolor display elements.
For example, Japanese Patent Application Publication No. 2003-121883 (hereinafter referred to as “Patent Document 1”) discloses an electrochromic display element having an organic electrochromic layer formed of polymeric microparticle layers.
Japanese Patent Application Publication No. 2006-106669 (hereinafter referred to as “Patent Document 2”) discloses a multicolor display element having different electrochromic compositions or different electrochromic layers that develop different colors.
Japanese Patent Application Publication No. 2010-33016 (hereinafter referred to as “Patent Document 3”) discloses an electrochromic display device having different electrochromic layers arranged corresponding to different display electrodes.
Further, Japanese Patent No. 3934420 (hereinafter referred to as “Patent Document 4”) discloses an electrochromic display element capable of reducing its switching time. The disclosed electrochromic display element includes a first nanostructured film formed of electric conductive metal oxides on the display electrode. The first nanostructured film is formed on the display electrode in order to retain the electrochromic compound. The disclosed electrochromic display element further includes a second nanostructured film formed of electric conductive metal oxides on the counter electrodes. The second nanostructured film is formed on the counter electrodes in order to retain a counter material, which is a material inducing a reversed-phase redox reaction from the redox reaction of the electrochromic compound. That is, the electrochromic display element disclosed in Patent Document 4 has the first and second nanostructured films having large specific surface areas for retaining the respective electrochromic compound and counter material, which may increase color developing or color dissipating efficiency and reduce the switching time of the electrochromic display elements. The nanostructured films formed of electric conductive metal oxides utilized in the electrochromic display element disclosed in Patent Document 4 are low resistance films appropriately doped at a metallic level having surface resistivity less than 1.0E+05 ohm/square and a film thickness of approximately 3 μm.
In the development of practical electronic paper utilizing the electrochromic display element, thin-film transistors (TFTs) may frequently be utilized as drive elements. In the TFT, pixel electrodes (i.e., counter electrodes) are arranged at microintervals. If the low resistance layer disclosed in Patent Document 4 is formed on such pixel electrodes prepared as above, the low resistance layer may need to be separately formed on each of the pixel electrodes arranged at microintervals.
In this case, if the low resistance layer is continuously formed over the pixel electrodes (i.e., covered with one low resistance layer) in order to simplify a complicated fabrication process, color interfusion or image blurring may occur due to electric charge diffusion at the time of driving the electrochromic display element to develop or dissipate colors. As a result, the color may not be developed in a desired shape or exhibit insufficient development, which may drastically degrade display quality of the electrochromic display element.
In general, the electrochromic display element often deteriorates due to erosion caused by the electrolyte coming in contact with metal or electric conductive metal oxides utilized as the electrodes (counter electrodes in particular), and hence, some kind of practical protective layers may be necessary for protecting the electrodes from such erosion.